Lead storage batteries have a grid as a support for plates so as to ensure current collection and keep active substances in accordance with the shape of the plate. In recent years, there is a high tendency to use lead-calcium alloys as the grid in view of the fact that they have good maintenance-free characteristics. The lead-calcium alloys have higher melting points and poorer productivity by a casting system than conventionally employed lead-antimony alloys. Accordingly, it is ordinary to form a network porous material for keeping an active substance by punching an alloy sheet or by an expanding system in which an alloy sheet is slit in large number and expanded or spread.
The lead-acid batteries of the calcium alloy type exhibit good maintenance-free characteristics but have several drawbacks. One such drawback is a poor recovery property after overdischarge. The term "recovery property after overdischarge" used herein is intended to means the capability of recovering a capacity when a lead-acid battery is allowed to stand after complete discharge and is subsequently charged. For instance, a lead-acid battery which is employed in motorcars is often in a fully discharged condition in order to fulfill car functions by application of an electric current, e.g. a room light or a small light may be kept on over a long time by mistakes such as the door kept opened partially or the light not switched off, or a number of auxiliary devices or accessories having computer circuits are mounted. When the current application is continued over a long time under these conditions, the calcium-type battery increases in impedance. This may not be recovered by means of an ordinary constant potential charger.
Moreover, if a motorcar would be difficult to start and the start would become possible by charge from other motor vehicles, the battery would be difficult to recover by charge during traveling. With a gas absorption-type closed lead-acid battery which is used as a power source for general electric appliances, it is not always re-charged after use and may be in a condition which does not allow recovery as discussed above.
The above phenomenon has been confirmed as ascribed to the fact that the lead-calcium alloy used as the grid is exposed to an electrolytic solution which has been diluted by discharge and has a composition close to water, and reacts with remaining lead dioxide to form, on the surfaces of the alloy, a layer of lead oxide (PbO.sub.x in which x=1-2) having a high impedance.
It is usual to use about 0.03-0.15 wt % of calcium in this type of lead-calcium alloy in order to impart a suitable degree of expandability. In addition, 0.3-3.0 wt % of tin is also added so as to impart ductility. In a range where the concentration of tin is low, the oxide layer becomes dense at the time of overcharge, the corrosion and intergranular corrosion resistances are good, and the alloy tends to embrittle. On the contrary, if the concentration of tin is too high, a suitable degree of hardness cannot be expected and the alloy cannot stand deformation when oxidized. For these reasons, it has been generally accepted that the use of tin in the above range is essential. However, the problem of overdischarge has been recently closed up and it has been made clear that the overdischarge problem cannot be overcome in the above range of the concentrations. Thus, the poor recovery under overdischarge conditions has been misunderstood as an inevitable drawback inherent to calcium-type lead-acid abtteries.